I have a people subscribing to a newsletter option on of my client's websites. Their addresses are stored in a MySQL table, i have successfully composed a mailshot using forms and BCC: mailed a "bulk" mailshot of about 10 test email addresses and it seems to be okay.

However, the real email list is already at 800+ addresses and growing quite rapidly.

I believe there is not real limit to how many addresses you can put into the BCC list for a mail; the problem is the more address you put into BCC, the longer it takes for the server to send the e-mail out. With 800+ people I can imagine it would get pretty slow. You might be better off using a CGI script to do what you're trying, but as far as I know what you're doing should work.